Fe3O4@C@MCM41-guanidine core–shell nanostructures as a powerful and recyclable nanocatalyst with high performance for synthesis of Knoevenagel reaction

In this study, preparation, characterization and catalytic application of a novel core–shell structured magnetic with carbon and mesoporous silica shells supported guanidine (Fe3O4@C@MCM41-guanidine) are developed. The Fe3O4@C@MCM41-guanidine was prepared via surfactant directed hydrolysis and condensation of tetraethyl orthosilicate around Fe3O4@C NPs followed by treatment with guanidinium chloride. This nanocomposite was characterized by using Fourier transform infrared spectroscopy, vibrating sample magnetometry, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, thermal gravimetric analysis, wide-angle X-ray diffraction and low-angle X-ray diffraction techniques. This nanocomposite have high thermal, chemical stability, and uniform size. Fe3O4@C@MCM41-guanidine catalyst demonstrated high yield (91–98%) to prepare of Knoevenagel derivatives under the solvent free conditions at room temperature in the shortest time. Also, this catalyst was recovered and reused 10 times without significant decrease in efficiency and stability. Fortunately, an excellent level of yield (98–82%) was observed in the 10 consecutive catalyst cycles.

Missouri, USA), Merck Chemical Co. (Darmstadt, Germany) and Fluka Chemical Co. (Buchs, Switzerland). Cetyltrimethylammonium bromide (99%), Tetraethyl orthosilicate (≥ 99%), 3-chloropropyltrimethoxysilane (≥ 97%), Resorcinol (≥ 99%), Formaldehyde solution (37 wt. %), Toluene dried (≥ 99.5%), Guanidinium chloride (≥ 99%), Triethylamine (≥ 99.5%) and Ethyl cyanoacetate (≥ 98%) were purchased from SigmaAldrich and used without further purification. Benzaldehydes (97-99%) and NH 3 (28-30% wt) were purchased from Merck. Ethanol (≥ 99.8%) was purchased from Fluka. FT-IR spectra were recorded in the range of 400-4000 cm −1 on Alpha Centaur FT-IR spectrophotometer using KBr pellets. The morphology of the synthesized NPs was investigated by using the EM3200 scanning electron microscopy (SEM) device. Transmission electron microscopy (TEM) image was recorded using a FEI TECNAI 12 BioTWIN microscope. TGA analysis was performed by a Netzsch STA 409 PC/PG apparatus in the temperature range of 25-900 °C. Powder X-ray diffraction (PXRD) analysis were carried out using D8 ADVANCE XRD and Philips XPert Pro XRD equipment, respectively. X-ray distribution analysis (EDS) was performed by the means of an EDS Sirius SD device. The magnetic effect of the synthesized NPs was investigated using vibrating sample magnetometer (VSM) of Meghnatis Daghigh Kavir Co. The uniform dispersion of the reactants was obtained using a KMM1-120WE301ultrasonication device. The TLC-Grade-silica gel-G/UV 254 Thin-layer chromatography (TLC) device was used to evaluate the reaction advancement and determine the reaction completion. The melting point was measured using a Barnstead Electro-Thermal device. Preparation of Fe 3 O 4 @RF. The magnetic Fe 3 O 4 NPs were firstly prepared according to reported procedures 10,56 . To prepare the Fe 3 O 4 @RF NPs, 0.7 g of Fe 3 O 4 NPs were added to the reaction vessel containing 200 mL of water and 100 mL of ethanol. Then the resulted mixture was placed in to the ultrasonic bath for 30 min at room temperature. In the following, 1.5 mL of NH 3 (28-30% wt) was added in the reaction vessel at room temperature and obtained mixture was again placed in the ultrasonic bath for 15 min. After that, 1 g of resorcinol and 0.1 mL of formaldehyde solution were added in the reaction vessel, and the resulting mixture was stirred at room temperature for 24 h. Then, the resulting precipitate was separated by using a magnetic field, washed several times with water and ethanol, dried at 60 °C for 6 h and was named Fe 3 O 4 @RF 57 .

Preparation of Fe 3 O 4 @C@MCM41.
To preparation of Fe 3 O 4 @C@MCM41, first 0.6 g of Fe 3 O 4 @RF NPs were firstly added into the reaction vessel containing 60 mL of distilled water and was placed into the ultrasonic bath at 40 °C for 30 min. Subsequently, 3 mL of NH 3 (28-30% wt) and 1 g of cetyltrimethylammonium bromide (CTAB) were added into the reaction vessel. Then, 0.7 mL of Tetraethyl orthosilicate (TEOS) was added dropwise, and the resulting mixture was stirred for 2 h. In the next step, the reaction mixture was stirred for 48 h at a temperature of 100 °C. After the end of the reaction, the obtained precipitate was separated by a magnetic field and dried. The acquired material was poured into a crucible and placed in a furnace with a temperature of 350 °C for 5 h for the remove of CTAB surfactant. Finally, the resulting material was poured into a crucible, carbonized at 600 °C for 6 h under argon atmosphere and named Fe 3 O 4 @C@MCM41 58 .  added into a flask reaction containing 20 mL of dry toluene under ultrasonic waves. Subsequently, 0.08 g of guanidinium chloride and 0.003 of Triethylamine were added into the flask, and the obtained mixture was refluxed condition for 28 h. After, the acquired precipitate was separated by using a magnetic field, washed with 20 mL of ethanol and dried at 50 °C for 8 h. and named Fe 3 O 4 @C@MCM-41-guanidine ( Fig. 1).

General procedure of performing the Knoevenagel reaction in presence of Fe 3 O 4 @C@
MCM-41-guanidine nanocatalysts. For this purpose, 1 mmol of Ethyl cyanoacetate, 1 mmol of Benzaldehyde derivatives, and 1.5 mol % of Fe 3 O 4 @C@MCM-41-guanidine catalyst were added into the round-bottom flask and reaction vessel placed into ultrasonic bath at room temperature under solvent free condition. The reaction progress was monitored by TLC. After finishing of the reaction, hot ethanol was added to the reaction mixture and the catalyst was separated by an external magnet. After, the remaining solution was placed into an ice and precipitate formed. The product was separated by a paper filter and dried under ambient temperature.

Results and discussions
Initially, the magnetic Fe 3 O 4 NPs were synthesized. In the next step, these NPs were covered using Resorcinol-Formaldehyde (RF) polymer layer. Then, the Fe 3 O 4 @RF was covered with the MCM-41 shell by using the Sol-gel method in the presence of CTAB surfactant under alkaline conditions. After calcination to remove CTAB and carbonization, the Fe 3 O 4 @C@MCM41 was treated with 3-chloropropyltrimethoxysilane and Guanidine (Fe 3 O 4 @C@MCM41-guanidine) (  (Fig. 2d,e). The wide appeared peak in the range of 3400-3500 cm −1 is related to the stretch connections of O-H and NH. The presence of these peaks verifies the successful consolidation and high stability of the expected functional groups during the stages of catalyst synthesis (Fig. 2).
The wide-angle X-ray diffraction (WXRD) analysis of Fe 3 O 4 , carbon and Fe 3 O 4 @C@MCM41-guanidine nanomaterials was showed in   (Fig. 4b). This result confirms the successful coating of the Fe 3 O 4 @C with guanidine modified MCM-41 shell without causing any damage in its mesoporous structure 21,64 .
The EDS spectrum showed that the Fe 3 O 4 @C@MCM41-guanidine nanocatalyst is composed of Iron, Silicon, Oxygen, Carbon, and Nitrogen elements confirming the successful incorporation/immobilization of expected species in the material framework (Fig. 5).
The Scanning electron microscopy (SEM) technique was used for determine the appearance and particle size of the Fe 3 O 4 @C@MCM41-guanidine nanocatalyst. This analysis confirms that the designed nanocatalyst has homogeneous and nearly spherical structure. Moreover, SEM image show that the particles of this nanocatalyst are between 33-93 nm in size (Fig. 6).
Transmission electron microscopy (TEM) analysis was employed to validate the appearance and shell-core structure of the Fe 3 O 4 @C@MCM41-guanidine nanocatalyst. The TEM image showed that the designed nanocatalyst has a core-shell structure with a black core (magnetite particles) and gray shells (Fig. 7).
The thermal stability of the Fe 3 O 4 @C@MCM41-guanidine catalyst was investigated using thermal gravimetric analysis. This analysis was performed in the temperature range of 25 to 900 °C. The weight loss in the first stage (up to 100 °C) indicates the extraction of water and existing organic solvents in the synthesis stages of the catalyst. In the second stage, the weight loss in the temperature range of 100 to 300 °C indicates the extraction of residual surfactant. The weight loss in the next stage which occurs in the temperature range of 300 to 480 °C shows the elimination of organic groups. In the last stage, the weight loss at the temperature range of 400 to 800 °C shows the extraction of the remaining organic groups. This analysis shows the presence and thermal stability of the groups that covered the surface of the Fe 3 O 4 magnetic NPs (Fig. 8).
The  (Fig. 9). Also, this proves the high magnetic properties of Fe 3 O 4 and Fe 3 O 4 @C@MCM41-guanidine nanomaterials, which are very important for their easy separation in the chemical processes.
After the synthesis of Fe 3 O 4 @C@MCM41-guanidine nanostructure, its catalytic activity was investigated in the Knoevenagel reaction. For this purpose, the reaction conditions were firstly optimized. To optimize the reaction condition, the condensation between benzaldehyde and ethyl cyanoaceteat was selected as the model reaction. The effects of temperature, catalyst loading, and solvent were investigated under ultrasonic conditions. Examination of different solvents showed that the reaction yield was very low in the presence solvents such as toluene and acetonitrile, and it was slightly better in polar solvents such as ethanol and water. The best result was obtained under the solvent-free conditions. Subsequently, the catalyst loading was optimized with 0.5, 1.5, and 2.5 mol% of catalyst. The best yield was obtained with 1.5 mol%. It should be noted that the increase of catalyst loading did not affect the reaction progress because the amount of raw materials corresponds to 1.5% of catalyst and the excess amount of catalyst will be practically useless. The catalytic effect of Fe 3 O 4 @C@MCM41-guanidine can be proved by noting that the catalyst-free reaction did not make much progress after a long time (Table 1).  Table 2, aldehydes with electron acceptors groups such as 4-nitro benzaldehyde (Table 2, entry 7), 4-chloro-benzaldehyde (Table 2, entry 2), 4-cyano-benzaldehyde ( Table 2, entry 8), have very good performance and have high efficiency in a short time. Also, electron donor aldehydes such as 4-methoxy benzaldehyde ( Table 2, entry 4), and 3-ethoxy-4-hydroxybenzaldehyde (Table 2, entry 6) have moderate and good performance and efficiency under this conditions. Moreover, aldehydes with space barriers such as 2-hydroxy (Table 2, entry 5) and 2-methyl ( Table 2, entry 3) have relatively good efficiencies, indicating the high performance of Fe 3 O 4 @C@MCM41-guanidine nanocatalysts in the Knoevenagel reaction (Table 2). It should be noted that according to previous studies, condensation between condensations of cyanoacetate with aromatic and aliphatic aldehydes leads to the formation of a product with E-isomer 45 .
To investigate the recyclability and reusability of Fe 3 O 4 @C@MCM41-guanidine nanocatalyst in the Knoevenagel reaction, 1 mmol of benzaldehyde, 1 mmol of Ethyl cyanoacetate, and 1.5 mol% of Fe 3 O 4 @C@MCM41guanidine nanocatalyst at room temperature were selected as model reaction under solvent-free condition and ultrasound waves. Upon completion of the process, the magnetic nanocatalyst was separated using an external magnet, washed completely with ethanol, and dried. Then the recovered catalyst was reused in the next reaction cycle under the same conditions as the first run. The results show that the catalyst can be reused at least 10 times without significantly reducing the reaction time and efficiency (Fig. 10).  www.nature.com/scientificreports/ In the mechanism of the Knoevenagel reaction, one of the active hydrogens of methylene is separated by a base. Then, a nucleophilic attack is performed on carbon aldehyde or ketones. As can be observed in the Fig. 11, the base catalyst separates the active methylene hydrogen from ethyl cyanoacetate. The corresponding enolate is formed with the role of a nucleophile, then the resulting enolate leads to the formation of the corresponding β-hydroxyl with simultaneous nucleophile attack to carbonyl and attracting protons from the catalyst. In the next step, the desired product is formed by removing the water.

Conclusion
In summary, in this study, Fe 3 O 4 @C@MCM41-guanidine nanocatalyst was successfully prepared and its catalytic performance was studied. FT-IR, EDX and TGA are shown successful stabilization of the guanidine groups on the surface of the Fe3O4@C@MCM41 nanostructure. SEM and TEM analysis confirms the spherical structure of the Fe 3 O 4 @C@MCM41-guanidine nanostructure. The WXRD proved the high stability of crystalline structure of Fe 3 O 4 NPs during the material preparation. The LXRD also confirmed the formation of mesoporous silica shell around Fe 3 O 4 @C NPs. The VSM analysis demonstrated good magnetic properties of this nanocomposite. The catalyst showed excellent activity in the Knoevenagel reaction. Catalyst was simply recycled and reused 10 times without significant reduction in activity.

Data availability
All data and materials are included in the manuscript.  www.nature.com/scientificreports/